Solar batteries that can directly convert sunlight into electric energy are expected as sources of alternative energy to oil. Among solar batteries, crystalline silicon solar batteries including crystalline silicon substrates have high photoelectric conversion efficiency and have already been widely put to practical use as solar power generation systems. Among crystalline silicon solar batteries, a crystalline silicon solar battery in which an amorphous silicon thin film having a band gap different from a band gap of monocrystalline silicon is formed on a monocrystalline silicon substrate surface to form diffusion potential is called hetero-junction solar battery.
Further, a solar battery in which a thin intrinsic amorphous silicon layer is interposed between a conductive amorphous silicon thin film and a crystalline silicon surface for forming diffusion potential is known as one of forms of a crystalline silicon solar battery having the highest photoelectric conversion efficiency. Because the thin intrinsic amorphous silicon layer is formed between the crystalline silicon surface and the conductive amorphous silicon thin film, it is possible to subject a defect (mainly a dangling bond of silicon) present on the surface of crystalline silicon to termination treatment with hydrogen while reducing generation of a new defect level due to the film formation. It is also possible to prevent carrier introduction impurities from diffusing to the crystalline silicon surface when the conductive amorphous silicon thin film is formed.
In recent years, from the viewpoint of material problems and costs of a crystalline silicon solar battery, there is an increasing demand for reducing the thickness of a monocrystalline silicon substrate in use. Therefore, a technique for efficiently confining light in a substrate is becoming important according to the decrease in the thickness of the monocrystalline silicon substrate. There is also an increasing demand for reducing a light absorption loss in a transparent electrode. However, when the carrier concentration of the transparent electrode is reduced in order to reduce the light absorption loss in the transparent electrode, the electric conductivity of the transparent electrode decreases. Therefore, compatibility of the light transmissivity and the electric conductivity of the transparent electrode is a problem.
Patent Literature 1 describes a solar battery including a transparent electrode formed on the surface of a photoelectric conversion layer and a collector electrode formed in a predetermined region on the transparent electrode. In the solar battery, the transparent electrode is formed in which a section in the vicinity of an interface with the photoelectric conversion layer and a section that corresponds to a region where the collector electrode is formed are set as a high conductivity region and the section other than these sections is set as a high light transmissivity region. However, in this form, the high conductivity region having a high light absorption coefficient is present in an optical path. Absorption of light increases, improvement of photoelectric conversion efficiency is small, and oxygen plasma treatment is performed to form the high light transmissivity region. Therefore, there is a problem in that electric conductivity is reduced by oxidation of silver, which is the collector electrode.
Patent Literature 2 discloses a crystalline silicon solar battery in which, by providing a region having high carrier concentration only in a region not substantially irradiated by incident light right under a collector electrode, a layer having high light transmissivity and having electric conductivity is formed in an optical path and a layer having high carrier concentration is present under the collector electrode. In this crystalline silicon solar battery, junction from a transparent electrode layer to the collector electrode is made satisfactory and photoelectric conversion efficiency has been improved.